1. Field of the Invention
The invention relates to the use of supercritical fluid processing techniques to manufacture a variety of orthopedic parts such as bone grafts that can be used as spinal implants.
2. Description of the Prior Art.
There is a need for orthopedic parts such as bone prostheses, spinal implants, and the like. For example, in the field of spinal surgery, removal of all or a portion of a damaged intervertebral disc requires that the resulting space be filled to prevent disc space collapse and to promote fusion of the adjacent vertebrae across the disc space. Desirably, the space will be filled with an implant that will have adequate strength to withstand loads imposed by the vertebrae and which will permit bone ingrowth. Although implants made of a metal such as titanium alloy have adequate strength, they have various drawbacks such as a tendency to have relatively long fusion times. Implants made of bone grafts are desirable because bone grafts are biological materials which are replaced over time with the patient""s own bone, via the process of creeping substitution. Over time a bone graft virtually disappears, unlike a metal implant which persists indefinitely.
Unfortunately, bone grafts present several disadvantages. Autogenic bone is available only in limited quantities. The additional surgery also increases the risk of infection and blood loss and may reduce structural integrity at the donor site. The graft harvesting surgery is alleged to cause extreme pain that may exceed the pain of the fusion surgery. Allogenic and xenogenic bone grafts are undesirable because they involve the implantation of a bone of foreign origin into the body, with attendant risks of infection or rejection. Desirably, a bone substitute would be available that would have the advantages of autogenic bone, without the drawbacks of allogenic or xenogenic bone.
In response to the foregoing concerns, the present invention provides a technique for the manufacture of orthopedic parts such as bone grafts that have the desirable characteristics of autogenic bone, but without the drawbacks of allogenic bone or xenogenic bone. Such parts can be provided with the porosity and strength characteristics of either cortical bone or cancellous bone, as required.
The invention includes a reactor that has a mixer, a mold that has a cavity of a desired shape, and a conduit that connects the reactor and the mold. The reactor is charged with starting materials that will produce an orthopedic part of desired strength characteristics. The starting materials include a source of calcium ions and a polymer or multiple polymers that forms a matrix for the calcium source. A process medium is added to the reactor. In the preferred embodiment, the process medium is liquid CO2. After the reactor is sealed, the process medium is heated and pressurized to form a supercritical fluid. The heated and pressurized ingredients are mixed in the reactor for a period of time sufficient to form them into a homogeneous, gas-saturated suspension, or supercritical fluid slurry. The supercritical fluid slurry then is transferred from the reactor into the mold through the conduit. The slurry solidifies quickly in the mold to form a strong, dense product having a porous structure. Typically, the product will have a high percentage of calcium, will be porous with 100% interconnectivity of pores (i.e., without isolated pores), and will have pore sizes on the order of 300-400 mircrons.
The parameter of the process and the equipment for carrying it out can be varied to produce products having different characteristics. For example, the starting materials and their relative proportions, the process medium used and the temperature and pressure in the reactor, the mixing time, the pressure drop that occurs during material transfer, the temperature of the conduit, the shape of the mold, and the temperature of the mold can be varied. If desired, an orifice or a nozzle having multiple openings can be disposed in the conduit to control the flow rate and the size of the particles. The size of the pores in the finished product can be controlled by varying the density of the supercritical process medium and by the rate at which depressurization occurs. By using the present invention, a variety of orthopedic parts that simulate autogenic bone and which have different porosity and strength characteristics can be obtained.